It is well-known that there is pressure to eliminate lead compounds from fuels for reasons of public health and environmental protection. Although the specifications for reformulated gasolines set by EPA will come into force in 1995, standards were brought into force on Nov. 1, 1992 requiring gasoline contain 2.7 wt % oxygen during the winter in nonattainment areas of the U.S. If the federal air quality standard for CO has not been achieved by a specified attainment date, the minimum oxygen content will increase to 3.1%. Moreover, starting in the summer of 1992, the maximum blending Reid vapor pressure (BRvp) of all gasolines is set at 9.0 psi. Since oxygenates are not only used as gasoline blending components, extenders, octane boosters and as key ingredients for reducing the emissions of CO and VOCs (Volatile Organic Compounds), it is expected that the demand for oxygenates will increase enormously in the coming years. See F. Cunill, et al., "Effect of Water Presence on Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether Liquid-Phase Synthesis". Ind. Eng. Chem. Res. 1993, 32, 564-569.
Of all oxygenates, the tertiary ethers, such as methyl t-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) are preferred by refineries to lighter alcohols. They have lower blending Ried vapor pressure (BRvp), lower vaporization latent heats and low solubilities in water. The most common ether in use today is MTBE with a production of about 25 million metric tons. However, ETBE is becoming more attractive as the price of methanol goes up in relation to gasoline. It can be produced from renewable ethanol, in contrast to methanol derived from natural gas, and its use would help mitigate the greenhouse effect, Ibid., p. 564.
In addition, ETBE outranks MTBE as an octane enhancer and its BRvp is only 4 psi, which makes it more attractive for BRvp blends less than 8 psi required in some places during the summer. Therefore, a number of U.S. states and European countries are planning to make ETBE from bioethanol, Ibid.
At the present time, TAME, which is usually produced in MTBE refinery units when C.sub.5 olefins are diverted into the feed, is not viewed as rivaling MTBE or ETBE, Ibid.
The main drawback of tertiary ethers, is that they substantially increase aldehyde emissions which are under EPA regulations and have to decrease 15% by 1995. It is believed this drawback could be largely circumvented by mixing the tertiary ethers with tertiary alcohols. Tertiary butyl alcohol (tBA) has a very low atmospheric reactivity and low aldehyde emissions, since no hydrogens are contained in the carbon link to the oxygen. Basis experience acquired with tBA during the 1970s, a gasoline blended with a mixture of ethers and tBA and/or tertiary amyl alcohol should be shippable, Ibid.
Currently all commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol (Eq. 1), catalyzed by a cationic ion-exchange resin (see, for example: Hydrocarbon Processing, October 1984, p. 63; Oil and Gas J., Jan. 1, 1979, p. 76; Chem. Economics Handbook-SRI, September 1986, p. 543-7051P). The cationic ion-exchange resins used in MTBE synthesis normally have the sulphonic acid functionality (see: J. Tejero, J. Mol. Catal., 42 (1987) 257; C. Subramamam et al., Can. J. Chem. Eng., 65 (1987) 613). ##STR1##
With the expanding use of MTBE as an acceptable gasoline additive, a growing problem is the availability of raw materials. Historically, the critical raw material is isobutylene (Oil and Gas J., Jun. 8, 1987, p. 55). It would be advantageous, therefore, to have a process to make MTBE that does not require isobutylene as a building block. It would be advantageous to have an efficient process for making MTBE by reaction of methanol with tertiary-butyl alcohol, since t-butanol (tBA) is readily available commercially through isobutane oxidation.
The preparation of methyl tert-butyl ether from methyl and tert-butyl alcohols is discussed in S. V. Rozhkov et al., Prevrashch Uglevodorodov, Kislotno-Osnovn. Geterogennykh Katal. Tezisy Dokl. Vses Konf., 1977, 150 (C. A. 92:58165y). Here the tBA and methanol undergo etherification over KU-2 strongly acidic sulfopolystyrene cation-exchangers under mild conditions. This reference contains data on basic parameters of such a process. It is also pointed out that, although a plant for etherification over cation exchangers does not present any major problems, considerations include the fact that recycling large amounts of tert-butyl alcohol and methanol, as well as isobutylene, causes the scheme to be somewhat more expensive. Also, the progress of the reaction over cation exchangers is usually complicated by various adsorption and diffusion factors, by swelling phenomena, and by the variable distribution of the components between the solution and ion-exchanger phase. Furthermore, said acidic cation-exchangers with an organic (polystyrene or polymethacrylate) backbone generally have a very limited stability range with regard to operating temperatures, with temperatures above 120.degree. C. normally leading to irreversible destruction of the resin and loss of catalytic activity.
In U. S. Pat. No. 2,282,469 to Frolich there is disclosed a process for preparing methyl tertiary-butyl ether over a catalyst comprising Kieselguhr impregnated with phosphoric acid at a temperature of about 175.degree. F. to 350.degree. F.
In U.S. Pat. No. 4,925,989 (1990), to Hagan et al., there is disclosed a process for preparing methyl tertiary-butyl ether wherein tertiary-butyl alcohol, isobutylene and methanol are continuously fed into a combination reactor distillation tower having a packed sulfonic acid resin catalyst bed where MTBE is produced.
G. B. Pat. No. 2,179,563 (1987) discloses the use of modified layered clay catalysts in reactions capable of catalysis by protons. Of particular interest in this invention were the three-layer sheet types, such as smectites, micas and vermiculites composed of successive layers of tetrahedral silica, octahedral alumina and tetrahedral silica which can exhibit swelling properties.
U.S. Pat. No. 4,590,294 (1986) discloses a process for the production of an ester comprising reacting an olefin from the group consisting of ethylene, hex-1-ene, hept-1-ene, oct-1-ene, 4-methylpent-1-ene, hex-2-ene, 1,5-hexadiene and cyclohexene with a carboxylic acid using as a catalyst component a hydrogen ion-exchanged layered clay. This reference would not seem to suggest a method for simultaneous dehydration of tert-butanol to isobutylene and the reaction with methanol to produce MTBE.
U.S. Pat. No. 4,822,921 (1989), To Texaco Chemical Co., discloses a method for preparing alkyl tertiary alkyl ethers which comprises reacting a C.sub.1 -C.sub.6 primary alcohol with a C.sub.4 -C.sub.10 tertiary alcohol over a catalyst comprising an inert support impregnated with phosphoric acid.
U.S. Pat. No. 5,099,072 (1992), to Texaco Chemical Co., discloses the reaction of t-butanol and methanol, or other C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols, in the presence of acidic montmorillonite clay catalysts having certain identifiable physical parameters, such as surface area, acidity range and moisture content.
In U.S. Pat. No. 5,059,725 (1991), to Texaco Chemical Co., a one-step synthesis for MTBE is disclosed wherein butanol and methanol are reacted over a catalyst comprising ammonium sulfate or sulfuric acid deposited upon a Group IV oxide. Again, other C.sub.1 -C.sub.6 alcohols and C.sub.4 -C.sub.10 tertiary alcohols will also work.
U.S. Pat. No. 5,157,162 (1992), to Texaco Chemical Co., discloses a fluorosulfonic acid-modified clay catalyst for the production of aliphatic ethers from C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols.
In U.S. Pat. No. 5,157,161 (1992), to Texaco Chemical Co., there is disclosed the use of a hydrogen fluoride-modified montmorillonite clay catalyst for producing alkyl-tertiary alkyl ethers.
U.S. Pat. No. 5,214,218 (1993), to Texaco Chemical Co., discloses a method for producing methyl t-butyl ether or other alkyl tertiary alkyl ethers using a catalyst comprising a montmorillonite clay treated with a haloacid.
It is known in the art to use pillared clays as catalysts in certain processes. Chem. Systems Topical Reports, Vol. II, 1986, Program, p. 61 (May 1987). Also see: European Patent Application 0083970A1 (Jul. 20, 1983) to British Petroleum.
Possible applications for pillared clays include catalytic cracking and olefin oligomerization and the reaction of alkylene oxides with alcohols. Chem. Systems, supra, p. 62.
Other reactions catalyzed by pillared clays include the hydration of olefins to alcohols and the reaction of olefins with acids to form esters. The clays can also be used in Friedel Crafts-type reactions.
There is a review of the use of pillared, cation-exchanged and acid-treated montmorillonite as catalysts for certain organic reactions by J. M. Adams et al., J. Inclusion Phenomena, 5, 663 (1987), and in Applied Clay Science, 2, 309 (1987). These clays display Bronsted and Lewis acid activities. It is noted that, while some cationic species are stable in solution over a wide concentration and pH range, others are not, particularly solutions containing aluminum. It is noted that it is difficult to ensure a reproducible Al.sup.3+ clay and moreover, since workers have used slightly different exchanging and washing procedures, a comparison between related experiments is hindered. Commercial acid-treatment is carried out using concentrated hydrochloric, sulphonic or phosphoric acids. The concentration of the acid and the time of the treatment is variable. Sometimes the excess acid is removed by washing, whereas in other products this is not the case. Therefore there is a great variety in the type and activity of acid-treated clays.
With the current interest in the production of MTBE as a blending component in high octane gasoline, the identification of catalysts which provide substantial yields and extended life is important in the art. A catalyst which provides substantial yields, permits the production of MTBE in one step and incorporates the added feature of phase separation of the product above a certain temperature should contribute substantially to the art.